This invention relates generally to optical fibers, and more specifically to optical fibers with polymeric coatings.
Various types of optical fibers are known to the art. One type of optical fiber which is of particular interest is GGP fiber (glass, glass, polymer), so called because it includes a glass core, a glass cladding, and a permanent polymeric coating or P-coat encircling the glass cladding. Optical fiber of this type is commercially available from the Minnesota Mining and Manufacturing Company (3M) under the VOLITION trade name.
A typical GGP fiber construction consists of a glass core, a reduced glass cladding (100 micron), a P-coat (125 micron) and 2 standard buffer coats (the first to provide microbend protection, the second to provide abrasion resistance), such as those available from DSM Desotech (DSM 3471-1-152A and 3471-2-136), to give a final diameter of 250 microns. By contrast, standard (non-GGP) fiber has a 125 micron glass cladding and 2 standard buffer coats (such as the DSM materials listed above) to give a final diameter of 250 microns.
The P-coat in GGP fiber is a cationically curable system which is typically based on epoxy resins. Other cationically curable resisn are also known, including those functionalized with cycloaliphatic epoxy groups or vinyl ethers. The P-coat, which typically contains iodonium hexafluoroantimonate, a cationic photoinitiator, is immediately applied to the fiber after the fiber is drawn from the furnace and is cured. Typically, one or more protective buffer coats are subsequently applied and cured over the P-coat to give the final GGP fiber construction.
Various other materials are also known which have been used as photoinitiators in various processes, materials, and systems, some of which are described below. Of those which have found use in fiber optic applications, most are radical photoinitiators, since the use of cationic photoinitiators in this area is still quite limited.
U.S. Pat. No. 5,668,192 (Castellanos et al.) discusses various iodonium borates as well as organometallic borates as photoinitiators. There is no mention of optical fiber in this patent.
U.S. Pat. No. 5,550,265(Castellanos et al.) discusses organometallic borates as photoinitiators. No mention of optical fiber coating or fiber strength.
GB 2307473 (Cunningham et al.) discloses organoboron photoinitiators of the generic formula G+xe2x88x92B (R1,R2, R3, R4), wherein G+ can be sulfonium or iodonium and R1, R2, R3 and R4 are alkyl groups. The photoinitiators are described as being suitable for photopolymerization of compositions with acid groups.
U.S. Pat. No. 4,854,956 (Pluijms et al.) describes a method for manufacturing optical fibers having a core and a cladding of glass applying a rod-in-tube technique. The reference discusses the use of HF etching solution and its effects on glass. The reference notes that, as fracture points often occur at points of contamination on the glass, HF solution can be employed to etch the quartz glass tubes. According to the reference, the conglomerates of alien particles are not attacked or hardly attacked, but the surrounding glass is attacked. After reaching a certain etching depth (10 micron) parts of conglomerates work loose from the surface with very low forces, such as the forces created in rinsing away the etchant.
U.S. Pat. No. 5,448,672 (Blonder et al.) discloses optical fibers with matte finishes. The authors use mixtures of buffered hydrofluoric acid (e.g., HF and NH4F) and a treating agent (acetic acid, phosphoric acid, sulfuric acid) to produce a matte finish on optical fibers, for purposes of reduced glare or improved adhesion. The background includes a reference to U.S. Pat. No. 4,055,458 which discloses the etching of glass by means of liquids containing HF.
U.S. Pat. No. 4,655,545 (Yamanishi et al.) discloses a glass fiber suitable for use in optical transmission. The reference discusses an optical fiber that has been extrusion coated with a fluorine containing resin, which are often found to have a mechanical strength which is much lower than fibers extrusion coated with non-fluorine containing coatings. The reference notes that xe2x80x9csuch a decrease in the mechanical strength is ascribed to fluorine gas or HF generated at the time of melt extrusion. More specifically, it is believed that fluorine gas or hydrofluoric acid generated during the extrusion coating passes through a first baked layer and reaches the surface of the glass fibers to erode the glass surfaces or destroy chemical bonding between the glass surfaces and the baked layer thereby causing the above-described reduction in mechanical strength.xe2x80x9d The reference proposes the use of an absorbable solid powder such as titanium oxide, calcium carbonate and the like to absorb the HF that is generated. 
Imides 
Methides 
Q=H, halogen, CN, R, aryl, Rf, RfSO2, RfCH2OSO2, (Rf)2CHOSO2
EP 834492 discloses the use of ionic compounds, such as those containing poly iodonium cations, as photoinitiators, although there is no mention of the application of these materials in coating optical fibers.
EP 775706, U.S. Pat. No. 5,807,905, and WO 9852952 disclose photoinitiators that have xe2x80x9cpolyboratexe2x80x9d anions.
U.S. Pat. No. 5,554,664 (Lammana et al.) discloses energy activated salts with fluorocarbon anions. The reference discusses the advantage of using catalysts with non-hydrolyzable anions for adhesives/coatings for electronics applications, because of the corrosiveness of HF that results from the hydrolysis of conventional initiator anions such as PF6 and SbF6, although the reference does not discuss the application of these materials as coatings for optical fibers. The reference focuses on methide and imide anions in onium salts, and also gives examples of initiators with borate anions.
PCT application WO 95/03338 discloses the use of salts of borates as polymerization catalysts. No reference is made to the use of the materials described therein in optical fibers.
EP 614958 discloses compositions with cationically crosslinkable polyorganosiloxane base and the use of these materials in the fields of anti-adhesion paper, fiber optics and printed circuit protection. Organometallic borate complexes are also disclosed having 4-10 groups with pi-bonded substituents (mesitlene, mesitylene, toluene etc) and borates with electron withdrawing groups, such as NO2, F, Cl, or Br.
U.S. Pat. No. 5,468,902 (Castellanos et al.) and U.S. Pat. No. 5,340,898 (Cavezzan et al.) discuss iodonium borate salts and cationically crosslinkable polysiloxanes.
While GGP fibers have been produced which have many admirable physical and optical properties, some of these properties have been observed to degrade under certain extreme conditions. In particular, some GGP fibers exhibit a decrease in fiber strength, as shown in dynamic fatigue tests, when they are placed in a high temperature/high humidity environment. Such environments may be replicated in an environmental chamber (FOTP-73) which is cycled from low to high temperatures over a period of approximately 10 days.
Unfortunately, such extreme conditions may be encountered by GGP fibers in applications such as avionics, naval or submarine operations, oil field applications, or even during manufacturing or shipping. Such conditions may also be encountered outside of the plant in areas where 85xc2x0 C. will likely be an upper specification temperature.
There is thus a need for a GGP fiber which exhibits greater strength retention after exposure to high temperature/high humidity environments. This and other needs are met by the present invention, as hereinafter disclosed.